This invention relates to a non-contacting optical gauging system and particularly to such a system which illuminates a target surface with a sheet of light which is evaluated to provide a characterization of the contour of the target surface.
Optical gauging systems are presently employed in industry for evaluating the profile shape of workpieces such as turbine blades, gears, helical threads, etc. These devices have inherent advantages over contacting-type mechanical gauges in that they can generally operate at greater speeds and are not subjected to mechanical wear due to direct contact with the workpiece. In one example of an optical gauge according to the prior art, a sheet of light is projected onto the object to be characterized. The illuminated portion of the object is viewed with a two-dimensional video camera or swept linear array along an axis at an angle skewed from the angle of the illumination beam. The sheet of light illuminates a profile of a cross section of the part which is viewed by the detector, just as if the part has been sliced along the light beam. Points nearer to the light source are illuminated to one side of the field-of-view of the detector, while further points are seen illuminated on the other side of the detector's field-of-view. Accordingly, such systems provide a means for evaluating the surface contour of a workpiece along a particular cross section of interest.
The above-described optical gauging techniques have a number of significant limitations. In many instances, it is desirable to evaluate a workpiece surface along a particular cross sectiom, such as perpendicular to the axis of symmetry of a turned workpiece, or along the chord of a turbine engine blade, etc. Characterizing such a cross section in one view normally requires the illuminating sheet of light to be brought in precisely along the plane of the specific cross section. To evaluate the workpiece contour, the camera views the surface at an angle from the plane of illumination with its axis intersecting the workpiece. This approach produces focus error aberrations, along with magnification errors across the surface (the so called "keystone effect") which complicates image processing. Moreover, viewing the plane of interest in this manner requires the viewing system to have a depth-of-field adequate to encompass the depth of interest, which imposes optical constraints and limitations on the measurement capacity and robustness of the data output of the system.
Alternate gauging approaches such as coordinate measuring machines (CMMs) obtain their accuracy by means of a high precision encoded translation stage which positions contacting probes. The operating speed of such systems is limited by the requirement that the machine stop and very slowly approach each measurement point so that the probes do not "crash" into the workpiece. Further, these systems have limitations in that the probe must touch the workpiece one point at a time in a serial manner. As a hybrid approach, noncontact triangulation probes have been attached to CMMs, but the measurements are still strictly done for selected individual points on the workpiece.
In view of the foregoing, there is a need to provide an optical gauging system which overcomes the depth-of-field and resolution limitations of prior art optical systems and which does not have focus error aberrations and magnification variations which can complicate data processing. It is further desirable to provide such a device which provides rapid gauging time and high measurement accuracy.